The mouse mammary tumor virus (MMTV) differs from most other oncornaviruses in that the mature virions are frequently produced throughout the animal's adult life. Most mouse mammary tumors have greater levels of MMTV expression and more MMTV DNA equivalents than the normal mammary epithelium. It is the purpose of this project to analyze the origins of the observed increases in MMTV genes and gene expression. Possible origin in a single cell (clonal proliferation) will be examined by restriction endonuclease mapping and other techniques.